Method for upgrading steel plant dust

ABSTRACT

The invention concerns a method for upgrading steel plant dust to recuperate industrially reusable compounds. Said method comprises an attrition and dilution step, a solid/liquid separating and solid washing step, an acid leaching step, and acid purification step comprising a carburizing reaction or a carburizing and decalcification reaction. Said method is characterised in that it comprises an oxidation/neutralisation step which includes a step oxidizing iron into iron oxyhydroxides and/or iron oxides and a neutralizing reaction of the reaction medium.

[0001] The present invention relates to a method for utilisingsteelworks dust.

[0002] The iron and steel industry, through its activity, produces wastein the form of dust.

[0003] During steel production in an electric furnace, this dust resultsfrom the following reaction.

[0004] Above the boiling melt, fine particles of iron are dispersed inthe vapour phase and carried along by the air intake. In this aircurrent, the particles are cooled and, under the action of the oxygen inthe air, are converted into higher oxides.

[0005] The most volatile metals pass into the vapour phase at theoperating temperature of the furnace. Under the effect of the intake,these volatile metals are oxidised and cooled by the air and end up inthe form of free oxides or structures mixed with the iron oxides.

[0006] The composition of steelworks dust depends on the nature of theiron and steel-making method. Steelworks dust contains variablequantities of majority elements such as iron, zinc, calcium and siliconin the form of simple or mixed oxides, and minority elements such ascopper, manganese, chromium, cadmium, lead and chlorides.

[0007] Steelworks dust therefore contains heavy metals which arenon-degradable over time and toxic to the living world.

[0008] Knowing that dust production is approximately twenty kilogramsper tonne of steel, that is to say of the order of 500,000 T/year inEurope, the reprocessing of this dust constitutes a real ecological andeconomic undertaking.

[0009] Up to now, steelworks have had to place, in a Class 1 wastedisposal site, their dust resulting from their activity in order for itto be stabilised. This forced movement to a waste disposal site broughtabout a non-negligible expenditure for the iron and steel industry.

[0010] The invention proposes to provide the manufacturers concernedwith an economically and ecologically more advantageous alternative.

[0011] This is because the invention aims to eliminate this waste byusing the dust from the iron and steel industry in a method enabling thedifferent components to be separated. The purity of said components isthen sufficient to enable them to constitute raw materials usable inother industries such as the mineral pigment industry among others.

[0012] Patent applications PCT/FR96/01202 and PCT/FR99/00813 constitutethe prior art of the invention.

[0013] Patent application PCT/FR96/01202 instances a method forpreparing mineral pigments from steelworks dust. This method comprisesthe following steps:

[0014] a) separation of the dust into two fractions, magnetic andnon-magnetic;

[0015] b) subjecting of the non-magnetic fraction to a basic lixiviationreaction;

[0016] c) rinsing, neutralisation and separation of the solid/liquidphases;

[0017] d) calcination between 450° C. and 650° C. of the charge;

[0018] e) treatment with sulphuric acid+catalyst of the calcined charge;

[0019] f) recovery of mineral pigments;

[0020] g) reuse of the solutions resulting from steps c) and e) in orderto precipitate other pigments.

[0021] As for patent application PCT/FR99/00813, this instances a methodfor treating steelworks dust with a view to the recovery of utilisableelements. This method comprises:

[0022] attrition in water;

[0023] hydraulic classification of the charge;

[0024] washing;

[0025] treatment under heat;

[0026] heat treatment between 240° C. and 800° C.;

[0027] treatment with sulphuric acid with a concentration between 5% and8%.

[0028] The methods described in the two patent applications constitutingthe prior art of the invention provided mineral pigments or utilisableelements with a purity unacceptable for the manufacturers using thistype of product.

[0029] The invention therefore aims to supply a solution by providing amethod for utilising steelworks dust making it possible to obtainmineral pigments or utilisable elements of sufficient purity to makethese products marketable.

[0030] The object of the invention is a method for utilising steelworksdust with a view to recovering industrially reusable compounds, saidmethod having an attrition and dilution step, a step of solid/liquidseparation and washing of the solid, an acid lixiviation step, an acidpurification step comprising a cementation reaction or a cementation anddecalcification reaction, said method being characterised in that itcomprises an oxidation/neutralisation step, said step comprising areaction of oxidation of the iron into iron oxyhydroxides and/or ironoxides and a reaction of neutralisation of the reaction medium.

[0031] According to one embodiment of the method, the oxidation reactionis carried out by means of an oxidant taken from the group comprisinghydrogen peroxide, oxygen and air.

[0032] According to one embodiment of the method, the neutralisationreaction is carried out by means of a solution of soda, potash, sodiumbicarbonate, potassium bicarbonate, ammonia or similar, in order tomaintain the pH of the reaction medium substantially between 2.5 and 5.

[0033] According to one embodiment of the method, theoxidation/neutralisation step is carried out substantially between 10°C. and 100° C.

[0034] According to one embodiment of the method, the acid lixiviationstep is carried out by means of a sulphuric acid solution titratingsubstantially between 8% and 25% acid and at a temperature substantiallybetween 50° C. and 100° C.

[0035] According to one embodiment of the method, the acid purificationis carried out at a pH substantially between 2 and 4 and at atemperature substantially between 20° C. and 100° C.

[0036] According to one embodiment of the method, a hydraulicclassification step is carried out after the attrition and dilution stepand before the step of liquid/solid separation and washing of the solid.

[0037] According to one embodiment of the method, said method comprisesa basic lixiviation step following the step of solid/liquid separationand washing of the solid.

[0038] According to one embodiment of the method, metallic zinc, zincoxides and/or zinc ferrite are recovered from the liquid phase resultingfrom the oxidation/neutralisation step.

[0039] According to one embodiment of the method, said method comprisesa step of drying and/or heat treatment of the solid phase resulting fromthe oxidation/neutralisation step enabling iron oxides and/or ironoxyhydroxides to be obtained.

[0040] According to one embodiment of the method, said method comprisesa step of electrolysis of the liquid phase resulting from theoxidation/neutralisation step, enabling metallic zinc to be obtained.

[0041] According to one embodiment of the method, the solid phaseresulting from the basic lixiviation step undergoes the acid lixiviationstep.

[0042] According to one embodiment of the method, the liquid phaseresulting from the basic lixiviation step undergoes a basic purificationstep.

[0043] According to one embodiment of the method, the basic purificationstep comprises a cementation reaction or a cementation and desilylationreaction.

[0044] According to one embodiment of the method, an electrolysis stepfollows the basic purification step, enabling metallic zinc and/or zincoxides to be isolated.

[0045] Other objects and advantages of the invention will emerge in thecourse of the description which will be given with reference to theaccompanying figures in which:

[0046]FIG. 1 is a schematic diagram depicting the essential steps of themethod for utilising steelworks dust in their order of implementation;

[0047]FIG. 2 is a schematic diagram depicting the steps of the methodaccording to one embodiment.

[0048]FIG. 1 depicts schematically the essential steps of the methodaccording to the invention, in their order of implementation.

[0049] The method for treating steelworks dust, which may contain Zn,Fe, Pb, Cd, Cr, Ca, Si and salts among other things, first comprises astep of attrition of raw dust in water.

[0050] The attrition in water makes it possible to break up the dustagglomerates which have a tendency to form. This step facilitates thesubsequent separation of the charge into two fractions:

[0051] dust of size greater than 40 microns;

[0052] dust of size smaller than 40 microns.

[0053] This moist phase treatment step assumes great importance in themethod and therefore constitutes, as such, an essential step as it makesit possible to:

[0054] speed up the dissolving of the soluble salts;

[0055] liberate the metallic fractions masked by the oxides adsorbed onthe surface.

[0056] The charge thus obtained is next diluted in water. This dilutionserves to wholly or partially solubilise the salts, such as chloridesand/or sulphates.

[0057] A step of separating the solid/liquid phases generated is thencarried out, which may be accompanied by washing of the solid phase. Theaim of this washing is to eliminate as much as possible the salts stillpresent in the solid phase where they were not solubilised in theattrition and dilution step. Finally, the solid phase is separated fromthe wash water.

[0058] This salt elimination step is important as it conditions, inpart, the degree of purity of the elements or compounds which will beisolated subsequently.

[0059] The solid phase may next undergo an acid lixiviation step. Forthis, the solid is treated with an acid solution, for example asulphuric acid solution, in order to totally or partially dissolve themetals such as iron, zinc, lead, cadmium, nickel and chromium. Theliquid phase, containing dissolved metals, is separated from the solidphase.

[0060] The silicon, which is insoluble, is contained in the solid phase.The acid lixiviation step has therefore made it possible, among otherthings, to separate the silicon from the other elements or compounds tobe utilised.

[0061] The liquid phase, which is acid, is next purified in order tototally or partially eliminate the metals such as lead, cadmium, nickeland chromium as well as the calcium, if necessary.

[0062] This acid purification step consists in adding, to the acidliquid phase, with a pH situated between 2 and 4, sodium fluoride inorder to precipitate the calcium if necessary, and powdered zinc and/oriron in order to cement the lead, cadmium, nickel and chromium.

[0063] The powdered zinc used in this purification step can be thatobtained by the method for the invention.

[0064] Cementation means deposition by reduction. During cementation, anoble body, oxidised and dissolved, is reduced by another metallic bodyadded to the solution in powdered form.

[0065] The liquid phase, resulting from the acid purification step, isthen recovered. It comprises the metals iron and zinc.

[0066] A step, including an oxidation reaction and a neutralisationreaction, said reactions capable of taking place simultaneously, iscarried out on the liquid phase comprising iron in particular. The aimof this step is to precipitate the iron in the form of ironoxyhydroxides and/or iron oxides, while the zinc remains in solution inthe reaction medium.

[0067] The oxidation reaction is carried out by means of a liquidoxidant such as hydrogen peroxide or a gaseous oxidant such as air oroxygen at a pH maintained between 2.5 and 5 by means of a basic solutionsuch as a soda solution or similar. This pH condition is important inorder to avoid the precipitation of the zinc at the same time as that ofthe iron.

[0068] At the end of this oxidation/neutralisation step, the followingare obtained:

[0069] directly, marketable products such as iron oxyhydroxides and/oriron oxides; and

[0070] indirectly, marketable products such as zinc oxides, zincferrites and metallic zinc, after treatment of the reaction medium.

[0071]FIG. 2 depicts a method for utilising steelworks dust according toone embodiment of the invention.

[0072] The description of the method shown schematically in FIG. 2 whichfollows will be accompanied by numerical values by way of example. Thesenumerical values are in no way limitative.

[0073] It will be considered for the remainder of the description thatone tonne of steelworks dust undergoes the utilisation method describedbelow.

[0074] The first step of the method for utilising steelworks dust is anattrition step. This moist phase treatment step is carried out in anattrition unit in the presence of water.

[0075] The ratio R1=(dry matter of steelworks dust in kg)/(volume ofwater in litres) must be chosen so as to allow a suitable frictionbetween the solid particles while providing a sufficient volume ofliquid to enable dissolving of the water-soluble fractions such assalts.

[0076] The ratio R1 is substantially between 2.2 and 2.7. This step iscarried out for 10 to 20 minutes.

[0077] The slurry, constituted by the attrited charge of steelworks dustmixed in the water, is next diluted in water in a manner sufficient to:

[0078] complete the dissolving of the salts, in particular the chloridesand sulphates;

[0079] put the very fine particles into suspension; and

[0080] provide a density adapted to hydraulic classification.

[0081] The diluting of the slurry makes it possible to achieve a drymass concentration substantially between 4% and 15%.

[0082] The slurry thus diluted next undergoes a hydraulic classificationstep. The hydraulic classification is advantageously carried out incut-point hydrocyclones adapted to the granulometry of the dustintroduced therein.

[0083] The slurry can pass through a hydrocyclone having a cut-offbetween 30 and 100 microns. The differential pressure is substantiallybetween 0.6 bar and 1 bar and the ratio R2=(incoming slurry flowrate)/(water counterflow rate) is substantially between 100 and 200.

[0084] At the end of the cycloning, the following are distinguished:

[0085] the underflow rich in magnetite and possibly comprising carbonand iron; and

[0086] the overflow rich in heavy metals, in the upper part of thevertical cylinder of the hydrocyclone.

[0087] The underflow enables recovery of 100 kg to 200 kg of dry matterwhile the overflow enables recovery of 800 kg to 900 kg of dry matterstarting with one tonne of treated steelworks dust.

[0088] The underflow charge can form the subject for example of arecharging of the furnace after decantation, spinning and drying or canform the subject of a utilisation of the magnetite fraction afterextraction.

[0089] The diluted slurry, or the overflow in the case where the dilutedslurry has undergone a hydraulic classification, next forms the subjectof a solid/liquid separation step. This separation can be performed, forexample, by filtration by means of a filter press. The solid phase isthen recovered on the filter. Simultaneously, the solid can be washed inorder to eliminate the salts which were not dissolved in the dilutionwater of the previous attrition and dilution step.

[0090] At the end of this solid/liquid separation and washing step, thechloride and sulphate concentration in the solid is less than around1000 ppm for each of the two salts.

[0091] The solid, obtained at the end of the solid/liquid separation andwashing step, can next undergo a basic lixiviation step. This step makesit possible to solubilise the free metals such as zinc, lead, cadmium,nickel and chromium, as well as silicon. On the other hand, iron is notsolubilised.

[0092] Depending on the origin of the dust, between 10% and 50% byweight of solid subjected to the basic lixiviation step is solubilised.

[0093] The bases which can be used are chosen from the group comprising:soda, potash and sodium bicarbonate. Other bases can advantageously beused.

[0094] In the case of soda, a concentration substantially between 240g/l and 450 g/l is required.

[0095] The ratio R3=(dry matter in kg)/(reaction volume in litres) canbe between 0.1 and 0.3.

[0096] The basic lixiviation reaction is carried out at a temperaturesubstantially between 70° C. and 120° C. The duration of the step is afunction of the quantity of compounds to be solubilised.

[0097] At the end of this step, a filtration of the solution enablesseparation of the solid and liquid phases. The solid phase, rich iniron, is directed to an acid lixiviation step while the liquid phase,rich in metals other than iron and possibly comprising silicon, can bedirected to a basic purification step which will be described a littlelater in the description.

[0098] Generally speaking, an acid lixiviation makes it possible tosolubilise all metals such as iron, zinc, lead and cadmium and to makesilicon insoluble.

[0099] Between 80% and 95% by weight of solid subjected to the acidlixiviation step is solubilised.

[0100] Where the basic lixiviation step is bypassed, the solid resultingfrom the solid/liquid separation and washing step undergoes an acidlixiviation step which leads to solubilisation of the iron, zinc, lead,nickel, chromium and cadmium it might contain, while the silicon remainsinsoluble.

[0101] Where the basic lixiviation step takes place, the iron containedin the solid phase obtained at the end of the basic lixiviation step issolubilised, as is the zinc tied up in ferrite form.

[0102] The acid used for the acid lixiviation step is, for example,sulphuric acid titrating substantially between 8% and 25% acid. Thisacid lixiviation step is carried out at a temperature substantiallybetween 50° C. and 100° C. The duration of the step depends on thequantity of compounds to be solubilised.

[0103] The ratio R4=(dry matter in kg)/(reaction volume in litres) issubstantially between 0.05 and 0.15.

[0104] At the end of the acid lixiviation step, the reaction medium is,for example, filtered on a filter press so as to separate the solidphase containing the insoluble compounds such as for example silicon,and the acid aqueous phase or filtrate containing iron and which mayalso contain the solubilised elements such as zinc, lead, cadmium,nickel and chromium.

[0105] The iron and zinc concentrations are substantially between 30 g/land 60 g/l for the iron and between 5 g/l and 20 g/l for the zinc.

[0106] An acid purification step follows the acid lixiviation step. Thispurification step comprises a cementation reaction and possibly adecalcification reaction.

[0107] The filtrate from the acid lixiviation step is purified bycementation of certain metals such as lead, cadmium, nickel and chromiumby the addition of zinc and/or iron. The quantity of zinc and/or iron tobe added is a function of the quantity of compounds to be cemented.

[0108] For one tonne of initial steelworks dust, up to around 200 kg ofzinc are necessary.

[0109] Cementation takes place at a pH substantially between 2 and 4 andat a temperature substantially between 20° C. and 100° C.

[0110] Decalcification can also be carried out, if necessary, duringthis acid purification step, simultaneously or not with the cementationreaction. In order to decalcify the filtrate from the acid lixiviationreaction, sodium fluoride is added to said filtrate in order toprecipitate the calcium.

[0111] At the end of this acid purification step, the reaction mediumis, for example, filtered on a filter press so as to separate the solidand liquid phases.

[0112] The filtrate resulting from the acid purification step has atotal concentration of lead, cadmium, nickel, chromium and calciumsubstantially less than 5 mg/l and on the other hand contains asignificant concentration of iron and zinc.

[0113] The solid phase resulting from the acid purification step can beneutralised and washed so as to achieve a pH substantially between 6.5and 7. This solid phase can be utilised subsequently as it containsheavy metals such as lead, zinc, etc in sufficient quantity.

[0114] The step following the acid purification step is the so-calledoxidation/neutralisation step. The aim of this step is to precipitatethe iron in the form of iron oxyhydroxides and/or iron oxides followingits oxidation.

[0115] To carry out the oxidation reaction, use can be made of a liquidoxidant of hydrogen peroxide type at a concentration substantiallybetween 5% and 35% or a gaseous oxidant of air or oxygen type.

[0116] It is imperative that this oxidation/neutralisation step becarried out at a pH substantially between 2.5 and 5. Adjustment of thepH into this pH range is made simultaneously with the oxidation reactionby the addition of a base such as soda at a concentration substantiallybetween 100 g/l and 200 g/l. The reaction is carried out at atemperature substantially between 10° C. and 100° C.

[0117] With the oxidation/neutralisation reaction complete, the reactionmedium is, for example, filtered on a filter press enabling separationof the solid phase containing iron oxyhydroxides and/or iron oxides andthe liquid phase or filtrate containing zinc at a concentration whichmay reach 50 g/l.

[0118] The solid phase recovered on the filter is washed and dried at atemperature substantially between 80° C. and 150° C., possibly followedby a heat treatment carried out at a temperature substantially between400° C. and 850° C. The iron oxides and iron oxyhydroxides obtainedafter drying and/or heat treatment constitute products marketable in thepigment field.

[0119] The filtrate resulting from the oxidation/neutralisation stepcontains essentially zinc. In order to utilise this zinc, three ways canbe envisaged:

[0120] electrolysis of the filtrate enabling isolation of metallic zinc;

[0121] synthesis of zinc ferrite;

[0122] obtaining of zinc oxides.

[0123] All these products constitute marketable products.

[0124] As mentioned above, the basic liquid phase resulting from thebasic lixiviation step can undergo a basic purification step. Thispurification comprises a cementation reaction, perhaps even adesilylation reaction, if necessary.

[0125] The basic liquid phase, rich in metals other than iron, iscleared of certain metals such as lead, cadmium, nickel and chromium bythe addition of zinc in sufficient quantity to cement all the metalsmentioned above.

[0126] This cementation reaction is carried out at a temperaturesubstantially between 20° C. and 100° C.

[0127] The desilylation reaction enables precipitation of the silicon.This reaction can be carried out simultaneously with the cementationreaction by adding slaked lime to the medium.

[0128] For example, in a particular composition, for one tonne oftreated steelworks dust, up to 9 kg of slaked lime is necessary.

[0129] At the end of this basic purification step, the reaction mediumis, for example, filtered on a filter press so as to separate the basicsolid phase and the basic liquid phase or basic filtrate.

[0130] The basic solid phase can contain metals such as lead, cadmium,nickel and chromium. It can also contain silicon. The basic solid phaseis neutralised and washed. It can form the subject of a utilisation ofthe metals it contains by subsequent treatments.

[0131] The basic filtrate, zinc-enriched, can undergo electrolysis. Theproduct, collected after electrolysis and with an acceptable faradaicefficiency, comes in the form of powdered zinc which, once washed anddried, makes it possible to obtain powdered metallic zinc and/or zincoxides, which are marketable products.

[0132] The electrolysed basic filtrate, zinc-depleted, can be reused inthe basic lixiviation step.

[0133] By way of example, without it being exhaustive, starting with onetonne of steelworks dust as the initial substance, the following areobtained:

[0134] 600 kg of iron oxyhydroxides and/or iron oxides at the end of thedrying and/or the heat treatment;

[0135] 300 kg of co-products.

[0136] The advantages of this method are:

[0137] being global since it aims to utilise all metals liable to bepresent in steelworks dust;

[0138] being economically and ecologically advantageous compared withplacing this type of waste in a Class 1 waste disposal site;

[0139] providing a better purity of the products thus utilised by themethod compared with the methods of the prior art;

[0140] making these utilised products marketable.

1. A method for utilising steelworks dust with a view to recoveringindustrially reusable compounds, said method having an attrition anddilution step, a step of solid/liquid separation and washing of thesolid, an acid lixiviation step, an acid purification step comprising acementation reaction or a cementation and decalcification reaction, saidmethod being characterised in that it comprises anoxidation/neutralisation step, said step comprising a reaction ofoxidation of the iron into iron oxyhydroxides and/or iron oxides and areaction of neutralisation of the reaction medium.
 2. A method accordingto claim 1, characterised in that the oxidation reaction is carried outby means of an oxidant taken from the group comprising hydrogenperoxide, oxygen and air.
 3. A method according to either of thepreceding claims, characterised in that the neutralisation reaction iscarried out by means of a solution of soda, potash, sodium bicarbonate,potassium bicarbonate, ammonia or similar, in order to maintain the pHof the reaction medium substantially between 2.5 and
 5. 4. A methodaccording to one of the preceding claims, characterised in that theoxidation and neutralisation step is carried out substantially between10° C. and 100° C.
 5. A method according to one of the preceding claims,characterised in that the acid lixiviation step is carried out by meansof a sulphuric acid solution titrating substantially between 8% and 25%acid and at a temperature substantially between 50° C. and 100° C.
 6. Amethod according to one of the preceding claims, characterised in thatthe acid purification is carried out at a pH substantially between 2 and4 and at a temperature substantially between 20° C. and 100° C.
 7. Amethod according to one of the preceding claims, characterised in that ahydraulic classification step is carried out after the attrition anddilution step and before the step of liquid/solid separation and washingof the solid.
 8. A method according to one of the preceding claims,characterised in that it comprises a basic lixiviation step followingthe step of solid/liquid separation and washing of the solid.
 9. Amethod according to one of the preceding claims, is characterised inthat metallic zinc, zinc oxides and/or zinc ferrite are recovered fromthe liquid phase resulting from the oxidation/neutralisation step.
 10. Amethod according to one of the preceding claims, characterised in thatit comprises a step of drying and/or heat treatment of the solid phaseresulting from the oxidation/neutralisation step enabling iron oxidesand/or iron oxyhydroxides to be obtained.
 11. A method according to oneof the preceding claims, characterised in that it comprises a step ofelectrolysis of the liquid phase resulting from theoxidation/neutralisation step, enabling metallic zinc to be obtained.12. A method according to claim 8, characterised in that the solid phaseresulting from the basic lixiviation step undergoes the acid lixiviationstep.
 13. A method according to claim 8, characterised in that theliquid phase resulting from the basic lixiviation step undergoes a basicpurification step.
 14. A method according to claim 13, characterised inthat the basic purification step comprises a cementation reaction or acementation and desilylation reaction.
 15. A method according to claim13 or 14, characterised in that an electrolysis step follows the basicpurification step, enabling metallic zinc and/or zinc oxides to beisolated.